Closed loop electronic control systems of the type contemplated herein are commonly used to control the temperature of a fluid or gas for any of a variety of purposes. For example, HVAC systems are used to control the temperature of air inside buildings and other enclosures designed for human occupancy. Refrigeration systems are typically used to store food, chemicals and other materials.
Many applications of temperature control systems require highly precise control of fluid or gas temperature. Because humans can sense changes in temperature of as little as 2.degree. F., HVAC systems must be able to precisely control the temperature of large volumes of air to maximize human comfort. To this end, various types of control systems have been employed in an attempt to provide well regulated control of temperature.
One type of closed loop control that has been used is a proportional controller. In general terms, proportional controllers operate to control a process variable on the basis of the difference between the measured or sensed value of that variable and a desired value of that variable. This difference typically takes the form of an error signal that is generated by subtracting the sensed value of the variable from the desired value (setpoint). This error signal is then used to operate an actuator or other controlled device that moves the process variable toward the setpoint. The sensitivity of these controllers to the error signal can be set as required by choosing an appropriate level of amplification of the error signal. Thus, the system can be made highly sensitive by a large amplification of the error signal. In many applications, this may provide suitable, accurate control of a process variable. However, increasing the sensitivity of a proportional controller will in most applications increase the instability of the system. Overshoots of the setpoint and oscillations can occur at frequencies related to the time constants of the variable as well as those of the control system. This can effectively limit the accuracy of a proportional controller.
Another disadvantage that is inherent in proportional control is that an error must exist between the sensed value and setpoint in order to produce a control output other than a predetermined initially set or calibrated value. Because the control variable or load in most systems require a continuous, but variable input to maintain the load at the setpoint, the system will inherently contain a certain amount of error between the sensed value and setpoint, colloquially known as "droop". This error may also appear as minor oscillations of the value of the controlled variable about the setpoint. However, in systems such as HVAC systems, such minor variations may be substantial enough that they can be felt by humans.
Controllers incorporating integral action are used to eliminate droop and to increase the sensitivity of the controller without decreasing system stability. These controllers are commonly referred to as proportional plus integral (PI) controllers. PI controllers eliminate droop because even small variations of the sensed value from the setpoint are integrated, thereby resulting in an increasing control signal which will bring the process variable back to the setpoint. An example of such a PI controller is disclosed in U.S. Pat. No. 3,946,297, issued Mar. 23, 1976, to J. H. Bechtel.
There are, however, several aspects of PI control that reduce accuracy and stability. The first, known as windup, occurs when the device that controls the system variable is driven to its maximum position or output in an effort to bring the system variable to its setpoint. If the system variable does not return toward setpoint (i.e., the load is greater than the control capability of the controlled device), the PI controller will continue to integrate the error signal and will thus, "windup". The controlled device will remain at maximum output until the system variable has crossed the setpoint and enough inverse error has been integrated to remove the integral windup. This effectively decreases the accuracy and stability of the system by causing large overshoots of the setpoint.
It is therefore desirable to eliminate windup in PI control systems. One such means for eliminating windup is disclosed in U.S. Pat. No. 3,938,017, issued Feb. 10, 1976 to T. E. Hayes. In that patent, proportional plus integral control is provided by independently generating the proportional and integral terms of the control signal, which are then summed together. The integral term is generated by an integrator circuit that includes a programmable operational amplifier. This programmable op-amp is a special type of op-amp in that it includes a control input for selectively disabling its operation. Windup is avoided by disabling the op-amp when it becomes saturated. Once the op-amp is disabled, the integral term is forced to zero and the control signal therefore provides only proportional control. The provision of separate channels for generating the proportional and integral terms of the control signal increases the size and complexity of the controller.
A second source of inaccuracy in PI controllers is the capacitive element (typically a capacitor) used to store the integrated value of the error signal. For HVAC and many other systems, integrating time constants on the order of minutes and parts of an hour are required to match the long time constants of air temperature changes. Otherwise, the controller will continuously over- and under-shoot the setpoint temperature. The time constant of the integrator portion of a PI controller is determined by the resistance and capacitance used in the feedback loop. That is, .tau.=RC. However, capacitors (including tantalum or aluminum electrolytics) having a capacitance over approximately 47 .mu.F allow a relatively large amount of current to flow through the capacitor. Such a current is known as leakage current and is substantial enough to undesirably effect integrator accuracy. The effect of leakage current can be modeled as a resistance connected across the capacitor which creates a voltage divider with the resistor used to determine the time constant (.tau.=RC). As a result, the feedback into the input of the integrating amplifier may be erroneously low and a correspondingly low integral will result. Thus, a capacitor having a high leakage current will reduce the ability of the system to accurately achieve the setpoint value of the system variable.
A solution to leakage current that is sometimes available is to chose a large value of resistance (R) and a small value of capacitance (C) for the time constant equation, since leakage current has a direct relationship to capacitance value. However, this solution is unavailable for very large time constants such as those required in HVAC controllers because it would require resistance values upwards of tens and hundreds of megohms. Such resistances may be greater than the resistance between traces on a printed circuit board or, because of moisture condensing from surrounding air, between individual components. Thus, there is a need for providing a large integration time constant using a resistance no more than approximately one megohm and a capacitor having a small leakage current.
Proportional and PI controllers typically require both positive and negative direct current power supply voltages. For controllers operable from alternating current supplies, such as the 24 volt alternating current standard in HVAC systems, this requires separate generation of both positive and negative voltages, which increases the size and complexity of controller designs. Thus, it would be desirable to provide a PI controller operable from a single supply (e.g., from positive voltages only).